The present description pertains to breathing assistance devices, such as ventilators, for example. Modern ventilators commonly employ positive pressure to assist patient ventilation. For example, after determining a patient-initiated or timed trigger, the ventilator delivers a specified gas mixture into an inhalation airway connected to the patient to track a specified desired pressure or flow trajectory, causing or assisting the patient's lungs to fill. Upon reaching the end of the inspiration, the added support is removed and the patient is allowed to passively exhale and the ventilator controls the gas flow through the system to maintain a designated airway pressure level (PEEP) during the exhalation phase. Other types of ventilators are non-triggered, and mandate a specified breathing pattern regardless of patient effort.
Modern ventilators typically include microprocessors or other controllers that employ various control schemes. These control schemes are used to command a pneumatic system (e.g., valves) that regulates the flow rates of breathing gases to and/or from the patient. Closed-loop control is often employed, using data from pressure, flow, and/or other types of sensors.
Certain ventilation configurations provide the potential for leaks occurring at various locations in the ventilation system, e.g., at connection interfaces between system components or at the interface between a breathing mask or tube and the patient's mouth or face. The magnitude of leaks in the system can vary from setting to setting, and/or dynamically within a particular setting, dependent upon a host of variables. Leaks can impair triggering (transition into inhalation phase) and cycling (transition into exhalation phase) of the ventilator, and thus cause problems with patient-device synchrony, undesirably increase patient breathing work, degrade advisory information available to treatment providers, and/or otherwise compromise the desired respiratory therapy.
FIGS. 1A and 1B provide an example pressure waveform and corresponding flow waveform to illustrate one effect of system leaks. FIG. 3A illustrates an example desired pressure waveform in a “Pressure Support” ventilation mode. FIG. 3B illustrates two different flow waveforms that provide the desired pressure waveform: one waveform in a configuration with no leaks, and one waveform in a configuration with leaks. As shown, during inspiration more flow is required in the leak configuration to achieve the same pressure level as compared to no-leak configuration. Further, during exhalation, a portion of the volume exhaled by the patient may exit through the leak and be missed by the ventilator exhalation flow measurement subsystem. In many cases, the goal of the control system is to deliver a controlled pressure or flow profile or trajectory (e.g., pressure or flow as a function of time) during the inspiratory phase of the breathing cycle. In other words, control is performed to achieve a desired time-varying pressure or flow output from a pneumatic system, with an eye toward causing or aiding the desired tidal breathing (e.g., the desired pressure profile shown in FIG. 3A).
Improper leak accounting can compromise the timing and magnitude of the control signals applied to the pneumatic system, especially during volume delivery. Also, inaccurate (or inadequate) leak compensation can jeopardize spirometry and patient data monitoring and reporting calculations. For example, the pressure applied from the pneumatic system to the patient may cause leakage of breathing gas to atmosphere. This leakage to atmosphere may occur, for example, at some point on the inspiratory limb or expiratory limb of the patient circuit, or at the connection between the breathing circuit and pneumatic system or between the breathing circuit and the patient interface (e.g., facial mask or endotracheal tube).
In the case of non-invasive ventilation, it is typical for some amount of breathing gas to escape via the opening defined between the patient interface (e.g., facial mask) and the surface of the patient's face. In facial masks, this opening can occur at a variety of locations around the edge of the mask, and the size and deformability of the mask can create significant leak variations. Due to the elastic nature of these masks and the movement of the patient, a leak compensation strategy assuming a constant size leak orifice (and thus constant leak flow) may be inadequate.
Accurately estimating and/or accounting for the magnitude of the leak flow may provide significant advantages. In order for a controller to command the pneumatic system to deliver the desired amount of volume/pressure to the patient at the desired time and measure/estimate the accurate amount of gas volume exhaled by the patient, the controller needs knowledge of the extent of the leak flow over time. The fact that the leak magnitude changes dynamically during operation of the ventilator introduces additional complexity to the problem of leak modeling.
Triggering and cycling (patient-ventilator) synchrony may also be compromised by sub-optimal leak estimation. In devices with patient-triggered and patient-cycled modalities that support spontaneous breathing efforts by the patient, it can be important to accurately detect when the patient wishes to inhale and exhale. Detection commonly occurs by using accurate pressure and/or lung flow (flow rates into or out of the patient's lung) variations. Leaks in the airway may cause an error in the signals to the sensors of the pneumatic system. This error may impede the ability of ventilator to detect the start of an inspiratory effort, which in turn compromises the ability of the controller to drive the pneumatic system in a fashion that is synchronous with the patient's spontaneous breathing cycles.
Accordingly, attempts have been made in existing control systems compensate for ventilation system leaks, which first involves estimation of the leak. However, prior techniques for determining leaks have had limited success, particularly in the case of patient-triggered breathing (i.e., patient-triggered inspiration).